1. Field of the Invention
Tunable lasers and systems for regulating the frequency output of monochromatic light sources find new expression in the invention, particularly for such purposes as frequency-shifting interferometry where interference patterns are produced at multiple measuring beam frequencies.
2. Description of Related Art
Light resonates within laser cavities between front and back facets in distinct frequency modes at which standing waves are produced by complete round trips taken by integer numbers of wavelengths between the facets. The potential for gain within the laser cavities varies as a function of frequency, and the optical power tends to concentrate in the frequency mode experiencing the highest gain or conversely the lowest loss. Beyond encounters with a lasing medium within the laser cavities, most other encounters of the light within the laser cavities entail losses, and the mode frequency experiencing the lowest loss is generally the one most amplified by the laser.
Frequency tuning of laser sources generally involves adjusting the conditions under which light is oscillated within the laser cavity to alter the nominal frequency that experiences the lowest loss. One way this is done is by coupling the output of the laser to an adjoining cavity that further participates in the oscillation of light. The external cavity includes the original cavity, which is filled with the gain medium and is referred to as “the lasing cavity”, and the adjoining cavity, which is not so filled and is referred to as “the feedback cavity”.
According to a so-called “Littrow” cavity configuration, the feedback cavity includes an adjustable facet in the form of a diffraction grating that diffracts one portion of the light (through a first order) on a path of retroreflection back toward the lasing cavity and reflects another portion of the light (through the zero order) in a second direction as the laser output. The lasing and feedback cavities are coupled together through a collimating lens, which collimates the light emitted through an active area on the front facet of the lasing cavity. The angle at which light is diffracted from the grating varies as a function of frequency. Of the diffracted light, only a limited band of frequencies are sufficiently aligned with the path of retroreflection to be focused by the collimating lens onto the active area of the front facet for reentry into the lasing cavity. By controlling the inclination of the diffraction grating, the frequencies capable of being retroreflected back into the lasing cavity can be adjusted.
The frequencies available for diffraction by the diffraction grating are limited to those that are amplified and emitted from the lasing cavity. The effect of returning any of the emitted frequencies to the lasing cavity is to alter the relative amounts of loss experienced among the emitted frequencies. A larger effect on the loss profile is produced by returning frequencies that are also capable of oscillating throughout the combined lengths of the lasing and feedback cavities with constructive interference. Losses are further reduced by the more limited set of frequencies that also constructively interfere between ends of the feedback cavity (i.e., between the front facet of the lasing cavity and the diffraction grating) by returning light to the lasing cavity at the same phase that the light left the lasing cavity. The frequency modes of the feedback cavity are generally more closely spaced than those of the lasing cavity.
Attempts to vary the frequency output of the laser cavities over a continuum are complicated by the effects of interference, which favors the oscillation of certain frequencies over others. To accommodate the effects of constructive interference generated by the feedback cavity, the gratings are generally translated in addition to being inclined through a range of angles so that the frequencies at which constructive interference takes place change with the frequencies that are retroreflected from the grating. However, if the front facet of the lasing cavity is highly reflective, the constructive interference within the lasing cavity still favors certain frequencies over others and as a consequence presents a risk of mode hopping, where abrupt changes in output frequency accompany much finer changes in feedback frequency. Often tuning is limited to frequency tolerance variations within a single lasing cavity mode.